4WD doesn't work
 

My friend has a '98 F150 XLT and recently had a local repair shop replace the shift motor unit, though the 4WD still doesn't work. The 4WD dash light never comes ON at all. Tested the 4WD mode switch and it appears to be okay. And the fuses were good. Looked at the electrical wiring diagrams and noticed there is a shift relay module located behind the center of the dash, but I couldn't find it. Please help me with this problem. thanks